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Delayed but not denied: North American monsoon to ramp up in southwestern US this week

By Renee Duff, AccuWeather meteorologist
July 22, 2019, 1:56:47 AM EDT


The anticipated uptick in showers and thunderstorms in the Southwest associated with the North American monsoon will bring welcome rainfall but also lightning, flash flood and dust storm dangers this week.

"One of the biggest weather events we see in the Southwest this time of year is the monsoon, but it has been lagging behind schedule this month," according to AccuWeather Chief Meteorologist Bernie Rayno.

The North American monsoon season begins on June 15 and ends on Sept. 30 each year.

Thunderstorms have been few and far between since the official start of the season, with most cities in the Southwest running precipitation deficits since its onset.


The monsoon officially began in Tucson, Arizona, on July 13, which is the city's latest onset since 2005, according to the United States Drought Monitor. The official onset at a certain location is based on average dew point temperature.

Rayno explained that a persistent dip in the jet stream has kept monsoon moisture bottled up over northern Mexico and dry air in place across the Southwest.

Dry air is an inhibitor of thunderstorm development.

This week, the jet stream will bulge northward as the pattern shifts. Heat will lift northward with the jet stream into the interior Northwest.

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"The flow aloft will become more southeasterly which will bring more moisture into the middle part of the atmosphere and create an environment that is more conducive to supporting thunderstorms," AccuWeather Senior Meteorologist and western U.S. blogger Brian Thompson said.

As is typically the case during the monsoon, storms will tend to blossom over the highest terrain during the afternoon hours and drift toward lower elevations into the evening and overnight.

"This increase in moisture will allow the showers and thunderstorms to develop over a wider area and impact areas outside of the mountains, so places like Phoenix and Las Vegas may start picking up the occasional drenching thunderstorm with heavy downpours and gusty winds," Thompson added.

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The downpours can trigger dangerous flash flooding in the arid region. Dry creek beds, known as arroyos, can suddenly fill with water due to a heavy thunderstorm upstream, overflow and flood neighboring roads and lands.

It is never a good decision to risk your life, the lives of other passengers and your would-be rescuers by driving through floodwaters.

Even in the absence of flooding, downpours will reduce visibility for motorists and create a heightened risk of hydroplaning while traveling at highway speeds. This includes on stretches of interstates 8, 10, 15, 17, 25 and 40.

Those hiking in the mountains should plan to head out early in the morning and return by midday to avoid the bulk of the storms. Be watchful for rapidly growing and darkening clouds which can be a sign that a storm is brewing.

If thunder is heard or lightning is seen, head immediately downhill to a valley or depression in the terrain.

In the absence of downpours, dry lightning strikes will elevate the risk of wildfires.

dust storm

(Photo/Getty Images)


"Dust storms will also become a possibility as some of the storms wander into the deserts," Thompson said.

For Phoenix, the risk for dust storms may unfold starting on Monday night.

Motorists should pull over on the side of a road with their emergency flashers on if a dust storm envelops the roadway as visibility can quickly drop to near zero and heighten the risk of multi-vehicle pile ups.

If driving conditions become too poor because of a dust storm, some roads may be closed for a time.

Download the free AccuWeather app for more details on when storms will impact your area using exclusive MinuteCast®. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.

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